diff --git a/python/langsmith/_internal/_serde.py b/python/langsmith/_internal/_serde.py
index d3c7d7d10..05d3c8857 100644
--- a/python/langsmith/_internal/_serde.py
+++ b/python/langsmith/_internal/_serde.py
@@ -79,6 +79,11 @@ def _simple_default(obj):
 ]
 
 
+# IMPORTANT: This function is used from Rust code in `langsmith-pyo3` serialization,
+#            in order to handle serializing these tricky Python types *from Rust*.
+#            Do not cause this function to become inaccessible (e.g. by deleting
+#            or renaming it) without also fixing the corresponding Rust code found in:
+#               rust/crates/langsmith-pyo3/src/serialization/mod.rs
 def _serialize_json(obj: Any) -> Any:
     try:
         if isinstance(obj, (set, tuple)):
diff --git a/rust/crates/langsmith-pyo3/src/py_run.rs b/rust/crates/langsmith-pyo3/src/py_run.rs
index 9725c5cd8..31ceb6cdf 100644
--- a/rust/crates/langsmith-pyo3/src/py_run.rs
+++ b/rust/crates/langsmith-pyo3/src/py_run.rs
@@ -22,6 +22,12 @@ impl RunCreateExtended {
 
 impl FromPyObject<'_> for RunCreateExtended {
     fn extract_bound(value: &Bound<'_, PyAny>) -> PyResult<Self> {
+        // Perform a runtime check that we've successfully located the `langsmith` Python code
+        // used to transform Python objects which aren't natively serializeable by `orjson`.
+        //
+        // This assertion ensures that we won't later fail to serialize e.g. Pydantic objects.
+        serialization::assert_orjson_default_is_present();
+
         let run_create = value.extract::<RunCreate>()?.into_inner();
 
         let attachments = {
@@ -58,6 +64,12 @@ impl RunUpdateExtended {
 
 impl FromPyObject<'_> for RunUpdateExtended {
     fn extract_bound(value: &Bound<'_, PyAny>) -> PyResult<Self> {
+        // Perform a runtime check that we've successfully located the `langsmith` Python code
+        // used to transform Python objects which aren't natively serializeable by `orjson`.
+        //
+        // This assertion ensures that we won't later fail to serialize e.g. Pydantic objects.
+        serialization::assert_orjson_default_is_present();
+
         let run_update = value.extract::<RunUpdate>()?.into_inner();
 
         // TODO: attachments are WIP at the moment, ignore them here for now.
@@ -273,7 +285,7 @@ fn extract_string_like(value: &Bound<'_, PyAny>) -> PyResult<String> {
             // However, orjson supports serializing UUID objects, so the easiest way to get
             // a Rust string from a Python UUID object is to serialize the UUID to a JSON string
             // and then parse out the string.
-            let Ok(buffer) = self::serialization::dumps(value.as_ptr()) else {
+            let Ok(buffer) = serialization::dumps(value.as_ptr()) else {
                 // orjson failed to deserialize the object. The fact that orjson is involved
                 // is an internal implementation detail, so return the original error instead.
                 // It looks like this:
diff --git a/rust/crates/langsmith-pyo3/src/serialization/mod.rs b/rust/crates/langsmith-pyo3/src/serialization/mod.rs
index 74972cad9..44458338a 100644
--- a/rust/crates/langsmith-pyo3/src/serialization/mod.rs
+++ b/rust/crates/langsmith-pyo3/src/serialization/mod.rs
@@ -1,21 +1,59 @@
+use std::ptr::NonNull;
+
+use pyo3::types::PyAnyMethods as _;
+
 mod writer;
 
+thread_local! {
+    static ORJSON_DEFAULT: Result<NonNull<pyo3_ffi::PyObject>, String> = {
+        pyo3::Python::with_gil(|py| {
+            let module = match py.import("langsmith._internal._serde") {
+                Ok(m) => m,
+                Err(e) => {
+                    let _ = py.import("langsmith").map_err(|_| "failed to import `langsmith` package; please make sure `langsmith-pyo3` is only used via the `langsmith` package".to_string())?;
+                    return Err(format!("Failed to import `langsmith._internal._serde` even though `langsmith` can be imported. Did internal `langsmith` package structure change? Underlying error: {e}"));
+                }
+            };
+
+            let function = module.getattr("_serialize_json").map_err(|e| format!("`_serialize_json` function not found; underlying error: {e}"))?.as_ptr();
+            Ok(NonNull::new(function).expect("function was null, which shouldn't ever happen"))
+        })
+    }
+}
+
+/// Perform a runtime check that we've successfully located the `langsmith` Python code
+/// used to transform Python objects which aren't natively serializeable by `orjson`.
+///
+/// This assertion ensures that we won't later fail to serialize e.g. Pydantic objects.
+///
+/// The cost of this call is trivial: just one easily branch-predictable comparison on
+/// an already-initialized thread-local.
+pub(crate) fn assert_orjson_default_is_present() {
+    ORJSON_DEFAULT.with(|res| {
+        if let Err(e) = res {
+            panic!("{e}");
+        }
+    })
+}
+
 pub(crate) fn dumps(ptr: *mut pyo3_ffi::PyObject) -> Result<Vec<u8>, String> {
     let mut writer = writer::BufWriter::new();
 
-    let obj = orjson::PyObjectSerializer::new(
-        ptr,
-        orjson::SerializerState::new(Default::default()),
-        None,
-    );
-
-    let res = orjson::to_writer(&mut writer, &obj);
-    match res {
-        Ok(_) => Ok(writer.finish()),
-        Err(err) => {
-            // Make sure we drop the allocated buffer.
-            let _ = writer.into_inner();
-            Err(err.to_string())
+    ORJSON_DEFAULT.with(|default| {
+        let obj = orjson::PyObjectSerializer::new(
+            ptr,
+            orjson::SerializerState::new(Default::default()),
+            default.as_ref().cloned().ok(),
+        );
+
+        let res = orjson::to_writer(&mut writer, &obj);
+        match res {
+            Ok(_) => Ok(writer.finish()),
+            Err(err) => {
+                // Make sure we drop the allocated buffer.
+                let _ = writer.into_inner();
+                Err(err.to_string())
+            }
         }
-    }
+    })
 }